144 GENERAL OBNITHOLOGY. 



of the centra of such vertebrae as are freely movable upon each other is highly characteristic 

 of birds, in so far as the shapes of the articular ends of the vertebral centra are concerned. 

 In anatomy at large, a vertebral centrum which is cupped or hollowed at both ends, is of 

 course bi-concave. Such a vertebra is called ampMcoelous (Gr. an^l, amphi, on both sides ; 

 KotXof, Jcoilos, hollowed) ; this is the rule in fishes, and obtained in some extinct Cretaceous birds, 

 as Ichthyornis ; it is unknown in recent birds.^ A centrum cupped in front only is proccelous ; 

 one cupped only behind is opisthoccelous (Gr. omaSe, opisthe, behind). Such structure neces- 

 sarily results in a ball-and-socket jointing of vertebrae. In those vertebrae of birds in which 

 this aiTangeinent obtains, it is always the posterior face of a centrum which is cupped, the 

 anterior one being balled; such vertebrae are therefore opisthoccelous. But in the freest 

 vertebral articulation of birds, that existing in the region of the neck, another modification 

 occurs. Both ends of each vertebra are saddle-shaped ; i. e,, concave in one direction, convex 

 in the other; a condition which may be called heterocoelous (Gr. erepos, heteros, conU'ary). 

 The concavo-convexity of any one vertebra fits the reciprocal concavo-convexity of the next. 

 Anterior faces of heterocoelous vertebrae are concave crosswise, up-and-down convex ; jjosterior 

 faces are the reverse ; consequently, such vertebrae are proccelous in horizontal section, but in 

 vertical section opisthoccelous. The various physical characters of vertebrae in different regions 

 of the body, and their connections with and relations to other parts of the body, have caused 

 their division into several sets, as cervical, dorsal, etc., which are best considered separately. 



Cervical Vertebrae (fig. 56, cv) are those of the neck : all those in front of the thorax or 

 chest, which do not bear free pleurapophyses in adult life, or the free pleurapophyses of which, 

 if any, are not in two-jointed pieces and do not reach the breast-bone; i. e., have no haema- 

 pophyses. It is advisable, in birds, to draw this line between cervical and succeeding vertebrae, 

 no other being equally practicable ; for, on the one hand, one, two or more of the cervicals 

 (recognizable as such by their general conformation and free articulation) may have long free 

 ribs, movably articulated ; and all the cervicals, excepting usually the first, or first and second, 

 have short pleurapophyses, anchylosed in adult life, but free in the embryo ; while, on the 

 other hand, a vertebra, apparently dorsal by its configuration and even its anchylosis with the 

 dorsal series, may be entirely cervical in its pleurapophysial character.^ Thus, in fig. 56, of an 

 owl's trunk, the bone which is apparently first dorsal, and is so marked (dv), bears a free 

 styliform " riblet " an inch long (c), only it is not jointed, and does not reach the sternum ; 

 while the next to the last cervical has a minute but still free rib (c). In a raven's neck before 

 me, the last cervical rib is about two inches long, articulating by well-defined head and shoul- 

 der to body and lateral process of the vertebra ; the penultimate rib is about half an inch long, 

 with one articulation to the lateral process ; while the next anterior vertebra (third from the 

 last) has a minute ossicle, as a free "riblet." The rule is two such free pleurapophyses or 

 cervical ribs of any considerable length : sometimes one ; rarely three ; in the cassowary four. 

 Rudimentary pleurapophyses may usually be traced up to the second cervical vertebra, as slender 



1 Except to this statement, however, the oddly-massed pygostyle, which, in birds where a terminal disc 

 develops inferiorly, may be distinctly cupped at both ends, as it is in a raven for example. 



- The case is very puzzling; the more so because, viewing the whole series of birds, the ambiguous "cervico- 

 dorsal," or two such equivocal vertebrae, may lean in different cases in opposite directions when the whole sum of 

 characters is taken into account. Therefore it may be best, as already said, to make the possession of a jointed 

 sternum-reaching rib the criterion of tlie Jirst dorsal vertebra, even though an antecedent one may have the 

 physical characters of a dorsal, and be anchylosed with the dorsal series. This is tlie view taken by Huxley, who 

 says: " The first dorsal vertebra is defined as such by the union of its ribs with the sternum by means of a sternal 

 rib." (Anat. Vert. Anim., 1872, p. 237.) Owen appears to regard as dorsal any of the vertebrae in question which 

 bear freie ribs. The actual uncertainty in the case, and the discrepant reckoning by diflferent authors, prevents us 

 from making a satisfactory count of the numbers of the two series of vertebrae in any given case. Thus, fig 56, as 

 marked by Dr. Shufeldt, shows siv dorsals (f/c), to which is to be added the one under 7;, bearing the rib sr; and 

 from which is to be subtracted the anterior one, bearing the rib c/, which is to be regarded as cervical, though its 

 physical characters are evidently those of the dorsal series. 



